You could talk about their life cycle, explaining how the eggs are fertilized, and the different stages of growth in the egg, hatching, and so on. I was interested in that kind of stuff when I was 15, though I was kind of a biology geek. Plus, baby chicks are adorable.
You can also talk about their importance in agriculture, and the different types, uses, and methods of raising them. I was in an Ag & Natural resources class, and everyone seemed to think the concept of a chicken tractor was really cool (eat bugs, fertilize soil, move, repeat). I assume girl scouts would have similar interests as kids in an ag class.
There are a lot of myths about chickens, I found it pretty cool when a teacher was talking about them. He asked us questions like
"Is a brown egg healthier than a white egg?"
And then, when someone answered, he explained that the color of the egg was related to the genetics of the chicken that laid it, and that the nutritional value of the egg was related to the hen's diet, not egg color. It keeps the audience thinking, and engaged.
There are a lot of myths like "chickens are filthy" though they actually spend much of their time preening their feathers.
Hope that helps some

Please teach them that, contrary to popular belief, chickens are not nasty creatures. They are also not stupid. They know and recognize their owners and flock members and can be quite affectionate and playful. They have different alarm calls. An alarm call for a predator in the sky is different from an alarm call for a predator on the ground. That way they know where to look for danger and how to react. Not all roos are dirt mean and looking for a fight. A good roo will find food for his hens, keep an eye out for predators, and often give his life defending his flock. Chickens are really fascinating. They each have their own personalities. Some are bossy, some are sweethearts, some are chatterboxes. Will your Girl Scouts be getting hands on experience with chickens?